The general aim of this project is to elucidate mechanisms by which proteins and viruses enter cells by receptor-mediated transport processes and translocate across the lipid bilayer to cytosol and nuclear compartments. Molecular signals which lead to unique vesicle trafficking are studied. Basic knowledge from these processes is used to design targeted drug delivery systems such as immunotoxins. Anti-human CD3 immunotoxins constructed with CRM9, a binding site mutant of diphtheria toxin, eliminate 80% of established tumors of human T cell leukemia in a nude mouse xenograft system. The cell killing power of the immunotoxin, when delivered at 1/2 the guinea pig minimum lethal dose, is equivalent to 500-600cGy from a 137Cs source and produces a 3 log kill of tumor cells. The therapeutic margin appears promising for clinic application which might include treatments for AM, autoimmune diseases and graft-versus-host-disease following bone marrow transplantation. Pre-clinical trials in nonhuman primates are now in progress. The diphtheria toxin receptor has been identified as belonging to the class of plasma membrane endosome recycling receptors. The plasma membrane potential sensitive step in vesicular stomatitis virus infection involves an early cytosolic pre-transcription step or transcription itself.